The present invention relates to a multi-direction time division multiplex (MD-TDM) communication system and, more particularly, to a MD-TDM communication apparatus for a central station of a MD-TDM communication system which is capable of regulating error rate despite deviations of timing points which may result from changes in temperature and other ambient conditions as well as from aging after optimum burst positions from satellite stations, or substation, to a central station have been set up at the time of initial system installation.
In a prior art terrestrial fixed radio MD-TDM communication system, a central station communicates with a plurality of remote satellite stations, which are distributed in a limited angular zone, using a broad-band beam antenna. The central station, as in ordinary digital fixed radio communications, sends to individual satellite stations time division multiplex (TDM) frame synchronizing signals and communication signals which are meant for the respective satellite stations in a broadcast mode. Each satellite station, on the other hand, extracts only the communication signal which is directed to the own station out of the received sequence of signals, while intermittently transmitting signals based on the frame synchronizing signals from the central station only in those time slots which are assigned to the own station. At the central station, the intermittent signals from the respective satellite stations are received in an orderly sequence as if they were transmitted from a single satellite station. The MD-TDM communication system in this manner achieves the so-called point-multipoint communication.
Generally, a terrestrial fixed radio MD-TDM communication system is free from fluctuation of daily period of a satellite and others which are observed in a satellite-aided TDMA system. In such a system, therefore, a receive clock to be applied to a code regenerator circuit, or ONE/ZERO decision circuit, of a central station to which an output of a demodulator is coupled is implemented with a central station reference clock, which is used as a reference for transmission at the central station. While a satellite-aided TDMA system is designed to extract a clock from a preamble which precedes a received burst so as to use it for ONE/ZERO decision, the terrestrial fixed radio MD-TDM system does not involve preambles as mentioned above and, thereby, cuts down redundancy to enhance effective use of frequencies.
In an apparatus installed in a central station of a prior art fixed radio MD-TDM communication system, a single code regenerator circuit is built in for discriminating ONEs and ZEROs of output signals of a demodulator which is adapted to demodulate received signals. In addition, only a single kind of clock is applied to the code regenerator circuit as a decision clock; the clock comprises either a central station clock or a signal which is prepared by delaying the phase of the central station clock by a predetermined time. The problem with such a system is that, while signals are sequentially routed through a transmitter at the central station, a receiver at a satellite station, a circuit at the satellite station adapted to set up a burst transmission timing to the cental station, a transmitter at the satellite station and a receiver at the central station, any fluctuation in the absolute delay time directly translates into a deviation in decision timing between a demodulated signal from the code regenerator circuit and the decision clock and, thereby, increases error rate. This leads to the need for various temperature compensation circuits capable of allowing the delay time to undergo a minimum of fluctuation against temperature variations, such circuits making the apparatus intricate in construction. In addition, careful temperature tests have to be performed with the equipment in addition to various kinds of troublesome adjustments.